The field of this invention comprises cylinders for printing presses and more particularly is concerned with the type of cylinders which would be used in a color-printing press that utilizes multiple cylinders, each required to provide a color component of a color image to a substrate.
Color printing as practiced heretofore is a well-known technology. Basically, a colored subject is photographed through separate filters to achieve so-called color separation negatives. These negatives are used to make half-tone plates to print the most common type of color printing utilizing black, yellow, cyan and magenta imprints to synthesize the color and texture of the original subject. The plates are mounted in letterpress or gravure machines and the substrate upon which the final imprint is to appear is run through the press with the plates all carefully synchronized to achieve perfect registration. Inks are chosen to achieve the desired results, these being applied to the plates and transferred from the plates directly or indirectly to the substrate.
Small runs of color prints or color printing effected with simple equipment can be done by printing sheets repeatedly with the different plates and inks in separate operations to achieve the desired results.
In recent years electrostatic imaging has replaced much of what was formerly done by photocopying of the so-calledphotostat type. By electrostatic imaging it is meant that the imaging is done by xerographic or electrofax techniques. In these techniques an electrophotographic member such as a sheet or drum having a coating of photoconductive material is charged by high voltage corona in darkness, the coating is exposed to a projected image of the subject matter selectively to discharge the same and produce a latent charge image which is a reproduction of the projected image and is then developed by toning.
In toning, fine particles of resin or other material is applied to the latent image surface. These fine particles are called toner in the art and are made up of different kinds of pigment, carbon, resin, dyes, etc. Toner is known in both liquid and powder form. In either case the particles are charged by triboelectric or electropheretic techniques to be charged to a polarity which will cause the particles to adhere to the photoconductive coating in accordance with the latent image. The latent image thus becomes visible.
In the case of xerography, where the electrophotographic member is for example a metal drum having a coating of amorphous selenium, the drum is pressed against a substrate such as paper and, in the presence of an electrical field is transferred to the paper after which it is fused to the paper by heat. In the case of electrofax, where the electrophotographic member is for example a sheet of conductive paper having a coating of zinc oxide particles in an inorganic matrix, the sheet itself is the finished article. The toner is fused to the sheet.
The principal kind of printing done by these techniques has been in single copies of some projected image. In the case of the xerographic apparatus, there is a new exposure and cycle for each copy. In the case of the electrofax there is also a new exposure and cycle for each copy but in this instance the electrophotographic member is not re-used but is removed from the apparatus as the copy.
The electrofax type of electrophotographic member has been used by suitable processing to form temporary printing plates by making the image and its background ink-differentiating. One is rendered hydrophobic and the other hydrophyllic by the process and in this condition the member is mounted in a printing press and used to make copies by treating it as a printing plate. Quality is low and the number of copies capable of being made from a single plate is substantially less than several thousand. Some attempts have been made by this method to achieve electrostatic color printing, but so far as known, there has been no success with this process because of many disadvantages, especially because zinc oxide plates are not panchromatic.
In U.S. Pat. No. 4,025,339 there is described a type of electrophotographic member and coating which is unique in its composition and properties. The coating is a sputtered coating of wholly inorganic material, for example, cadmium sulfide, deposited under certain condition in an r.f. sputtering chamber upon its substrate in a thickness of the order of 3000 to 6000 Angstroms. The coating displays high gain, electronic anisotropy, can be charged and discharged at high speed, has resolution easily attaining1000 line pairs per millimeter, is transparent, has a hard, crystalline structure with the crystals oriented vertically, has a dark resistivity of at least 10.sup.12 ohm centimeters and a ratio between dark and light resistivity of at least 10.sup.4.
This electrophotographic member is described as deposited onto a substrate of polyester having a thickness which is a fraction of a millimeter with an intervening bonded layer of some ohmic material such as indium-tin oxide of a thickness which is of the order of 200 Angstroms. This electrophotographic member can be used to make transparencies and has properties which enable it to perform as well as many types of silver film and better than most. It is highly flexible, being readily rolled around a rod less than a centimeter in diameter without damaging the coating. The coating is archival in quality since it is not adversely affected by heat, light, moisure or fungus.
The invention herein contemplates a novel use for the coating which, as explained in said U.S. Pat. No. 4,025,339, readily bonds to metal surfaces. The novel use is in making a printing plate, especially for use in color printing presses.
In U.S. Pat. Nos. 2,287,122 and 3,354,519 a type of printing cylinder is described which comprises an electrodeposited or electroformed sleeve of nickel that is a small fraction of a millimeter in thickness. The cylinders of this construction which are known are about a fifth of a meter in diameter and several meters long. Up to the present time these cylinders were used in a manner which required their manufacture with fine perforations. These perforations were made with patterns or images to provide for the expression of ink through the cylinder from the interior thereof by doctor means onto the substrate over which the cylinder rolled. Overall uniform perforations were applied in some cases and the patterns or images produced by blocking some of the perforations selectively.
The advantages of these cylinders are explained in the said U.S. Pat. Nos. 2,287,122 and 3,354,519, but basically they are economical, light in weight, easily transported, easily handled and mounted.
Problems arise in connection with these cylinders for use with electrostatic printing techniques which, it is believed, would direct one skilled in the printing art to avoid such use. These problems include the need for perforations and expressing inks through the cylinders from the interior thereof; the difficulties of causing known photoconductors to adhere to the cylinders and not craze, flake off, break or wear out; the need for applying excessively thick coatings to achieve reasonable imaging properties whereby the advantages of the thin walls and flexibility of the cylinders are obviated.
The invention herein resolves around a marriage of the thin-walled electrodeposited cylinder and the extremely thin and flexible coating of the U.S. Pat. No. 4,025,339 to achieve never before attained advantages in a printing cylinder.